A VCSEL is a semiconductor laser in which a first multiplicity of semiconductor layers (e.g., Group III-V compound layers) forms an active region (e.g., an MQW active region), which is sandwiched between a second and third multiplicity of layers, which form a pair of mirrors. One mirror, the bottom mirror, is formed under the active region and nearer the substrate, whereas the other mirror, the top mirror, is formed above the active region and farther from the substrate. The mirrors define a cavity resonator having its longitudinal axis oriented perpendicular to the plane of the layers. When the active region is forward biased and pumping current is applied thereto in excess of the lasing threshold, the VCSEL generates stimulated, coherent radiation that is emitted along the resonator axis. The wavelength of the radiation is determined by the bandgap of the material used to form the active region. Thus, for operation at relatively short wavelengths in the range of about 800-1000 nm, the layers of the active region typically comprise GaAs/AlGaAs compounds epitaxially grown on an optically absorbing GaAs substrate, whereas for operation at longer wavelengths of about 1100-1600 nm, the layers typically comprise InP/InGaAsP compounds epitaxially grown on an optically transparent InP substrate.
The radiation may emerge through either or both mirrors depending on their reflectivity. A VCSEL is termed a bottom-emitting device if the primary, relatively high intensity, emission is through the bottom mirror. This emission will propagate through the substrate if it is optically transparent. In many designs the substrate is removed and hence, even if it had been optically absorbing, does not obstruct the emission through the bottom mirror. On the other hand, the secondary, much lower intensity, emission that leaks through the top mirror is termed the backside emission. Bottom-emitting VCSELs are attractive because they are known to facilitate flip-chip bonding. In contrast, a VCSEL is termed a top-emitting device if the primary emission is through the top mirror.
One important feature of VCSELs is their ability to be fabricated in an array containing, for example, thousands of lasers. These arrays can be used to provide a multiplicity of carrier sources in fiber optic communication systems; e.g., dense optical interconnect solutions for high-end routers, cross-connects and switching systems. Before an array can be employed in a communications application, or any other application for that matter, it must be tested in order to determine whether each VCSEL, as well as the overall array, satisfies predetermined performance specifications. Defective VCSELs (i.e., those that do not meet specification) result in lower efficiency and wasted power consumption. Optimally an array is tested at a time in the manufacturing process (e.g., before substrate removal or final assembly) that minimizes economic loss should the array fail to meet specification and have to be discarded. To this end in the prior art, a top-emitting VCSEL array is probed in step-and-repeat fashion, one VCSEL at a time—a very time consuming, expensive process. The probe includes driver circuitry for supplying the necessary bias voltage and pumping current to the VCSEL under test and photodetection circuitry for measuring the intensity of the primary emission. Testing bottom-emitting VCSEL arrays is more problematic. For short wavelength bottom-emitting devices, the presence of the absorbing substrate prevents making optical measurements of the primary emission. To our knowledge, therefore, manufacturers limit their testing of short wavelength, bottom-emitting VCSELs to making electrical measurements to identify shorts or open circuits in each VCSEL, again using a step-and-repeat approach. In contrast, the substrate of long wavelength bottom-emitting VCSELs is transparent, but these devices are not currently in commercial manufacture to our knowledge.
Thus, a need remains in the art for an effective technique for testing bottom-emitting VCSELs regardless of their wavelength of operation.